Examples of such power supply apparatus are disclosed in JP 2001-35682A and JP 2006-337512A.
A power supply apparatus disclosed in JP 2001-35682A is for use with a discharge lamp and includes a converting circuit for converting an input supply voltage to a DC voltage. The output voltage of the converting circuit is applied to a buck converter which drives the discharge lamp. In order to ignite the discharge lamp, a high voltage from the converting circuit is applied across the discharge lamp, and, once the discharge lamp is ignited, the output voltage of the converting circuit is set to a value between 1.5 times and 2.5 times the lamp voltage of the operating discharge lamp on the basis of detection of the voltage across the discharge lamp. The power supply apparatus includes also an auxiliary power supply which supplements the converting circuit for its shortage of output voltage. With this arrangement, a voltage required for igniting the discharge lamp is secured, while improving the circuit efficiency during the steady state operation of the discharge lamp.
A power supply apparatus disclosed in JP 2006-337512A, too, is used to operate a projection lamp. In this power supply apparatus, commercial AC power is rectified in a rectifying circuit, and the rectified power is supplied through a power factor correction circuit to a primary winding of a transformer forming part of a switching regulator. The projection lamp is supplied with a voltage through this switching regulator.
The power supply apparatus disclosed in JP 2001-35682A obtains the discharge lamp igniting voltage from part of the power for the discharge lamp. Therefore, the voltage control is difficult because, when an input voltage to the power supply apparatus changes, the igniting voltage also changes. A lamp, e.g. xenon lamp, having an operating voltage in the steady state of several tens of volts, requires an igniting voltage five through ten times as high as the steady state voltage, and, therefore the power supply for the steady state operation of the lamp is not useable for igniting the lamp. On the other hand, if a power supply which can generate a high voltage for sustaining arcing is used, current loss and other disadvantages occur, which results in lowering of the efficiency, because such a high voltage is not necessary for the steady state operation. As disclosed in JP 2001-35682A, an auxiliary power supply may be used to provide a high voltage, but such auxiliary power supply requires large-sized diodes having a high-withstanding voltage, which inevitably results in increase of the cost and size of the power supply apparatus. Furthermore, when an auxiliary power supply apparatus arranged as disclosed in JP 2001-35682A is used to ignite the lamp, current abruptly flows through a load when igniting the lamp, resulting in voltage decrease, which, in turn, makes the operation of the lamp unstable, possibly leading to shortening of the life of the lamp.
The power supply apparatus disclosed in JP 2006-337512A is not provided with means, e.g. a pulse transformer, for ignition, but uses one transformer during steady state operation and ignition, and, therefore components of the switching regulator including the transformer must be able to handle large power, leading to increase of cost and size of the power supply apparatus.
An object of this invention is to provide an inexpensive and small-sized power supply apparatus for a load generating an arc.